So based on reading the advice in other threads on the subject I think I'm okay just to go with my tap water and adjust as necessary. Correct me if I'm wrong but my most important adjustments would be to Ca, SO4, and HCO3.

I'm going to be brewing an ESB and just as an exercise I popped these values (for the tap water) into Beersmith and compared it to London water. If I added a gram each of Gypsum, Table Salt, Epsom Salt, and Baking soda it would get me very close to that water profile with the exception of Sodium which would still be 41 ppm lower than that water profile.

Also I've seen many recommendations to add Campden tablets to the water to eliminate Cloromides (sp?). Does this have an impact on any of these numbers?

Any thoughts on the profile and my assumptions (actually wild ass guesses) would be appreciated.

Your analysis and profile looks pretty good (along with your assumptions, etc). Go to the Brewing Network and find the series of podcasts from Brew Strong about water. There's a wealth of knowledge there that will help to explain things better than a forum thread. Also, get a copy of Palmer's Residual Alkalinity spreadsheet to do your calculations. Shoot for RA vs the classic brewing water profile.

Your Britta filter has some ion exchange resin in it which removes hardness from the water. I see a lot of Ca and Mg missing while potassium and, to some extend, sodium increased. Definitely use the tap water or filter the tap water through a standard charcoal filter. Those filters don’t change the water profile.

In general, this is great brewing water.

When using Palmer’s spreadsheet, keep in mind that his SRM to RA formula doesn’t work so well for really dark beers which have a lot of roasted malts in the grist. Those beers need a lower RA that what you would expect from his spreadsheet. I know he says the opposite, but that doesn’t match my experience. I’m currently in the process of compiling a SRM / RA chart myself. It’s taking some time b/c the relationship is not linear and dependent on a number of factors. However, for any given beer color there is a rather wide residual alkalinity range that will work. Conversely, for a given RA there tends to be a wide range of beer colors that will work and will produce an acceptable mash pH. This is why many commercial brewers don’t even worry that much about water if it is of moderate hardness which you water is.

Make sure you don’t overdo the salt additions. Some brewers tend to get carried away with that and end up adding too much which may cause a mineral taste in the beer.

Thanks for the advice. I am planning on using a very light hand on the brewing salts until I get a better feel for what I'm doing.

One thing that occurred to me though is the calculations that I did to figure out ppm where based on the final recipe volume, in my case 5 gallons. If, say 1g of gypsum was added to the mash this would cause the ppm to be higher in the mash than in the final product since the mash water is only 3 gallons or so. Is that something that needs to be factored into the calculation of how much of a particular salt to use?

I'm thinking of the gypsum specifically because I think I need to at least bump up by Ca a bit for the mash, correct?

One thing that occurred to me though is the calculations that I did to figure out ppm where based on the final recipe volume, in my case 5 gallons. If, say 1g of gypsum was added to the mash this would cause the ppm to be higher in the mash than in the final product since the mash water is only 3 gallons or so. Is that something that needs to be factored into the calculation of how much of a particular salt to use?

The intention, with the spreadsheet you are using, is to treat the complete volume of brewing water with the calculated amounts of salt. But I doubt that it will make a big difference if you add all the salts to the mash and none to the sparge water. The pH may be a bit lower since there is more pH lowering Ca in the mash, but you didn’t calculate the RA and pH that precisely anyway.

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I'm thinking of the gypsum specifically because I think I need to at least bump up by Ca a bit for the mash, correct?

The importance of Ca in the mash is overrated. But it is generally accepted that a minimum amount of 50 ppm Ca in the brewing water is beneficial for a number of process steps in brewing. This being said, I have made very good and clear beer with just 20ppm Ca in the water.

If the original 19th century pilsners used any water treatment it would have been gypsum AFAIK. It's one of the things that made pale lagers such a recent development. Until well-modified pale malts became available you simply couldn't brew a beer that light in color. So people in places like Pilsen with extremely soft water more or less had to wait on the malts to be developed before they could get reasonably good conversion in a mash. They didn't know what pH was but they had access to hydrometers.

I have often wondered how the brewers in Pilsen get conversion with their soft water? Do they add some CaCl2?

the need of Ca for mash conversion is overrated. In fact the benchmarkmash, the congress mash, is done with distilled water. Ca stabilizes a-amylase but not b-amylase. At mashing temps a-amylase is plenty stable and it is b-amylase that could use some help.

What about adding Campden tablets? Should I still hit this water with a crushed up tablet before I use it?

That really depends on whether your water is treated with chlorine, chloramines, or both. The Brita filter will strip out chlorine but not chloramines. Your water utility will tell you; it may even be on their website.

From the advice given above I think that using the tap water and staying away from the Brita is the way to go.

My county water authority's website if fairly weak which is the reason I went with the Ward Lab report in the first place. A search of the entire website for "chloramines" yielded one notice about how consumers might notice a slight increase in chlorine due to a temporary switch from chloramines to chlorine during a routine water flushing exercise.

So from that I take it that they routinely use chloramines and I should use the campden.